Wrong. THey help carve definition between the different quad muscles.
well, that is true. but you gotta have a big piece of granite before you can start thinking about carving a statue

Dizzle, try doing the Trevor Smith fella workout on the leg extension 
I guarentee you will love it and your quads will get drastically better (this is specially tru for fellas with long legs )
i can do the close to the full stack with one leg on just about any leg extension machine for sets of 10. its an ok exercise but ill get into specifics why i dont like it down below
Leg extensions are a good exercise and will add good size.
Tbombz train 3x2 won't build any muscle.
training 3X2 might not add a whole bunch of size from one workout. but youll never add a whole bunch of size from one workout. training super hevay, low reps, low volume will boost strength considerable, so my next workout i can use that heavy ass weight again and get a cfew more reps. if i add in more sets i will be breaking down my muscle, not stimulating growth. stimulate, dont annihilate! -lee haney (but he could have benefited from training heavy and focusing on size instead of trying to pump his muscles with lighter weights)
guys... big muscle=strong muscle. and that means you can not have a big muscle unless it is first a strong muscle. you want size? you gotta get strong first. focus on lifting super heavy weights with good form, and dont train so much that your sore for days and dont gain strength. . just a few very heavy sets shy of failure. and hit it again in a couple days. once you are lifting 400-500 lbs on every major exercise, then focus on adding in reps, but only go as fast as your body will let you, dont try to force growth by destroying the muscle, going to failure, drop sets, etc. all this does is cause excess muscle damage and destroy your ability to make significant growth after your workout.